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We review standard mediation assumptions as they apply to identifyingantibody effects in a randomized vaccine trial and propose new study designs toallow identification of an estimand that was previously unidentifiable. Forthese mediation analyses, we partition the total ratio effect (one minus thevaccine effect) from a randomized vaccine trial into indirect (effects throughantibodies) and direct effects (other effects). Identifying $\lambda$, theproportion of the total effect due to an indirect effect, depends on across-world quantity, the potential outcome among vaccinated individuals withantibody levels as if given placebo, or vice versa. We review assumptions foridentifying $\lambda$ and show that there are two versions of $\lambda$, unlessthe effect of adding antibodies to the placebo arm is equal in magnitude to theeffect of subtracting antibodies from the vaccine arm. We focus on the casewhen individuals in the placebo arm are unlikely to have the needed antibodies.In that case, if a standard assumption (given confounders, potential mediatorsand potential outcomes are independent) is true, only one version of $\lambda$is identifiable, and if not neither is identifiable. We propose alternativesfor identifying the other version of $\lambda$, using experimental design toidentify a formerly cross-world quantity. Two alternative experimental designsuse a three arm trial with the extra arm being passive immunization(administering monoclonal antibodies), with or without closeout vaccination.Another alternative is to combine information from a placebo-controlled vaccinetrial with a placebo-controlled passive immunization trial.

Mediation Analyses for the Effect of Antibodies in Vaccination